1. Field of Invention
The present invention relates to a method for controlling light source. More particularly, the present invention relates to a method for controlling the light source brightness of a backlight device.
2. Description of Related Art
Before being dispatched from the factory, light-emitting diode (LED) backlight devices should be adjusted to the optimum by the manufacturer. However, the property of LEDs varies along with temperature and service life, such that the driving current designed by the manufacturer for driving the LED cannot make the backlight device to generate desired luminous effect. Therefore, many patents directed to solve the above problem are issued, such as U.S. Pat. No. 6,894,442, U.S. Pat. No. 6,127,783, and U.S. Pat. No. 6,495,964.
FIG. 1 is a circuit block diagram of the conventional backlight device. Referring to FIG. 1, a backlight device 100 comprises a light source module 110 having a plurality of LED light sources 112 and a frequency-varied pulse width modulation (FPWM) unit 122 for generating a PWM signal Vp to drive the light source module 110.
In the conventional backlight device 100, a detection module 130 is used to detect the luminous brightness of the light source module 110, and generates a current Id signal to an analog-to-digital converter (ADC) 144 via a limited current circuit 142. In general, the detection module 130 has a plurality of light detectors 132.
When receiving the output of the limited current circuit 142, the ADC 144 converts the analog current signal into a digital control frequency signal D1 and sends the digital control frequency signal D1 to the control unit 146. Then, the control unit 146 converts the digital control frequency signal into an analog control voltage Vc and sends the analog control voltage Vc to an input end of a frequency comparator 148. At this time, the frequency comparator 148 compares the control voltage Vc with a comparison voltage Vs from a variable resistor 152 connected in series to the light source module 110 and sends the comparison result to the FPWM unit 122.
The FPWM unit 122 generates the PWM signal Vp according to the output of a phase lock loop (PLL) 150. The FPWM unit 122 adjusts the duty cycle range of the PWM signal Vp according to the output of the frequency comparator 148. As shown in FIG. 2, C1 indicates an original PWM signal. When the brightness of the light source module 110 is weakened, the control voltage Vc is less than the comparison voltage Vs. At this time, the FPWM unit 122 increases the duty cycle of the PWM signal Vp as shown by C2, so as to increase the driving current of the light source module 110. On the contrary, the FPWM unit 122 can also reduce the duty cycle of the PWM signal Vp as shown by C3, so as to reduce the driving current of the light source module 110.